When the evenings get particularly thick with mosquitoes where I live, I sometimes sit out in the yard with my daughters and look up at the fading sky. Before too long, a single bat will usually flit out of the nearby trees and start flying circles around the house, scooping up bugs along the way. We can barely make out the bat’s wings as it takes its laps, a flicker of membranes. And so it was a revelation to spend some time earlier this week with two Brown University biologists, Dan Riskin and Sharon Swartz, watching slow-motion movies of bats in flight. There’s a lot going on up there.

Bats evolved about 50 million years ago from squirrel-like ancestors. They probably made their first forays into the air as gliders. Like living gliders, they used flaps of skin to increase their surface area, letting them glide further. Their hands evolved long spindly fingers that were joined by membranes. Some early bat fossils suggest that they may have shifted from gliding to alternating between gliding and bursts of fluttering. Eventually bats evolved sustained powered flight.

Bats evolved a way to take advantage of the same laws of physics birds use to fly. And many scientists who have studied bat flight in the past have basically treated bats like leathery birds. Yet there’s no reason to assume that this should be so. After all, it would not be surprising to find that the way the feathers on a bird’s wing react to air pushing against them are different from the way the stretchy membranes on a bat react. Birds don’t have wing surfaces connecting their front and back legs, like bats do. And while birds only have a couple joints in their wing skeleton, such as at the elbow and wrists, bats have lots of knuckles they could, in theory, bend selectively to alter their wing surface. Bats also have lots of sensitive hair cells on their wings that appear to track the speed and direction of the air flow, and the information they get from the hairs may help them make fine adjustments to their wings many times a second.

And when scientists like Swartz and Riskin study bats, they discover, in fact, that bats are not birds. Bats fly more slowly than birds, but they maneuver more effectively. Bats fly cheap compared to birds. A hovering bat use 60% less energy than a hovering hummingbird. These sorts of discoveries suggest that if you’d like to make an agile, efficient, and tiny flying robot (and who doesn’t?) it might be worth looking for some inspiration in bats.

The problem with looking to bats for inspiration is that scientists are only starting to figure out bat aerodynamics. What’s really challenging to figure out, however, is the difference between the aerodynamics of birds and bats. Riskin and Swartz use lots of tools to find the answer. They paint bright dots on bats and then film the animals as they fly in wind tunnels. The biologists can then use computers to create models of the bat wings and calculate the speed and direction of each dot at each instant of flight. They can spray mist into a tunnel and then film the swirls the bats leave in their wake. From this data on real bats, bat researchers can then test out simulations on computers to see if they produce the same forces and swirls of air as they see in their wind tunnels.

A close look at these movies reveals that bat flight is just too complex for simple labels, like upstroke and downstroke. The shoulder of a bat starts rotating upwards before the wrist, which move up before the fingers. The fingers on each hand don’t move in sync with each other. A joint on the left wing is often out of sync with the corresponding joint on the right wing.

Physicists like to treat wings as rigid surfaces because the math involved causes fewer headaches. But that’s a gross simplification when it comes to bats. The bones in a bat’s hands are surprisingly flexible, and the skin of the bat wing is never fully stretched out during its stroke. In fact, the region of the wing close to its body actually balloons out to double its surface area during each flight stroke. Bats probably use this ever-changing wing surface to control their lift and drag, so that they can make tight maneuvers without stalling.

Bats have clearly evolved a sophisticated flight system, but they face some awkward challenges when they’re not flying. Birds only need two limbs for flying, leaving their remaining two relatively free to land and walk around on the ground. Bats, on the other hand, make their hind legs part of their wings, and so natural selection has to strike a compromise between several different functions. And while birds can stop flying by using their feet to land on the ground, most bats have to use their feet to hang upside down.

To figure out how bats manage this feat, Riskin turned the typical biomechanics lab upside down. Scientists can measure the forces of a running animal by putting a force-sensitive plate on the floor; Riskin put his on the ceiling. In his recordings of landing bats, he’s discovered two strategies. In one species that lives in caves, the bats make an elegant backwards flip combined with an upside-down cartwheel, so that they can land with just two feet.

In a species that hangs from trees branches, the bats use a very different technique. They swoop in without a cartwheel, and bring both feet and both hands upward to grab onto the tree. And they hit the tree hard. The cave bats land with a force that’s twice their body weight; the tree bats generate forces as high as eleven times their body weight.

This discovery (published this week in the Journal of Experimental Biology) illustrates an important fact about bats–a bat is not a bat is not a bat. Bats live in many environments and are adapted to eating many different kinds of food, from moths to fruit to cow blood. They’ve adapted to these different ways of making a living, in part by evolving different ways of moving around. If you’re a bat flying towards a wall of rock, you don’t want to hit it too hard. But if you can grab a branch that can absorb the shock, you can skip the fancy acrobatics.

That same lesson emerges from how bats behave on the ground. With their delicate legs yoked together by their wings, you might expect that bats don’t do very well on the ground. And indeed, most species won’t win any track and field medals. When Riskin puts a typical bat on a treadmill, they stumble around. If the treadmill goes too fast they start to lose all control. It’s likely, then, that the ability to walk efficiently and to run was lost in the early evolution of bats. But millions of years later, that ability evolved once more in at least two species.

One place where bats have taken to the ground again is New Zealand. The remarkable isolation of New Zealand left it without big predators and without any mice or other ground-dwelling mammals. One species, the New Zealand short-tailed bat, has adapted to this niche. While it can still fly, it now moves around comfortably on the ground in search of bugs, nectar, fruit, and pollen.

Riskin found that New Zealand short-tailed bats walk comfortably on a treadmill, using the same pendulum-like movements that other walking mammals use to save on energy. But when other mammals have to move faster, they break into a run so that they can store extra energy in their tendons as they hit the ground. The New Zealand short-tailed bat can’t make the transition from walking to running.

But another species of bat can make that switch. A vampire bat will walk on the ground to sneak up on its victim. If its victim tries to get away, it can scramble in pursuit. Riskin found that if he put vampire bats on treadmills, they can walk like New Zealand short-tailed bats. But when he speeds up the treadmill, they suddenly switch to a bizarre form of running. Instead of pushing off with their hind legs, like a squirrel, they use their long, heavily muscled arms. It’s a mammal version of front-wheel drive versus rear-wheel drive.

The difference between the two species of ground-moving bats is not surprising when you consider where they live. Bats on New Zealand didn’t pay any cost for evoling into slow walkers, because life was pretty easy (at least before humans showed up with their rats and other assorted camp followers). But vampire bats evolved in a more competitive environment where they had to adapt to moving prey.

Once bats evolved flight, in other words, they did not stop evolving. Their movements have been changing in astonishing ways for millions of years, and will continue to change as long as bats fly, walk, or run across the Earth.

This is absolutely incredible in terms of your topic here and your articles are very well written too. I have often wondered if we should look at the physics of animal/biology movement and form a lot more, not only to learn more about animals but to know about new physics per se, how to apply it to AI as an example, or human flight, or just to figure out new things.

Is the flying bat dynamically unstable, like a modern jet fighter, or just very versatile?

And I wonder if there are any insights from the bat that could help the design of mobility aids for disabled people? I see the walking bat starts with the left foot forward like any other mammal such as a horse or cat but the running vampire bat gait resembles a lame person on crutches who is moving quickly.

William–Sorry you’re having trouble. It seems to work fine for me. But this is a work in progress! YouTube and our blogging system don’t always play nice. I’d prefer actually to use Vimeo for higher resolution, but when I tried embedding videos from there, it was a total disaster.

Katie–I saw them there too. I thought, “This demands blogging.” Fortunately, I was already planning a trip to Brown for a talk, where I was able to also sit down with Riskin and Swartz and get the big picture.

Amazing! And though hardly scientific, I am intrigued/disturbed by the fact that the bat in the final video is running the same way I do in those wierd dreams where one cannot get somewhere fast enough by walking!

Ok, got it. I guess you’re referring to the outliers in Fig 5 of the paper. Interestingly, maximum downward (as opposed to upward) force produced by a cave bat is a little more than four times body weight.

>Is the flying bat dynamically unstable, like a modern jet fighter, or just very versatile?

Ross,
The stable/unstable distiction really only applies to fixed wing aircraft. If it flies upright and level without control inputs then it’s stable. You could even extend the idea to insects that fly with relatively fixed patterns of wing movement.

Bats and birds are like modern fighter aircraft in that they need constant instantaneous control of the wings to get where they’re going.

I’m curious whether there are any bat species which are proficient at gliding. Most large birds that fly like to glide as much as possible, which they can do because their wings present as an airfoil.

And what about riding updrafts and thermals, like vultures? Are there any carrion bats?

[Carl: I’ll defer to any bat experts reading the Loom, but I think you need to distinguish between gliding that some small mammals do–which are really just controlled falls–and the soaring that a vulture or an albatross does. Bats probably evolved from gliding mammals, and they can still glide (see here, for instance), but only interspersed with flapping.]

I am surprised by several things. First, that bats are more economical flyers that birds, although I suspect again this depends on which species of bird and bat you compare. The reason I assumed bats were less ecomincal is because they rarely glide – strange considering they may have evolved from gliding squirrels. Compare the charicatured frantic flapping of a bat to a red kite, which stays aloft with just a few flicks of its tail and a favorable wind.
Second, I was surprise by the use of the hind legs. In the first video you can clearly see an in-and-out motion of the legs, controlling the adjacent wing surfaces. In the hovering video, the hind legs appear relaxed and flap around all over the place! But clearly bats have evolved to fly using all four limbs, instead of two like a bird.
[Carl: Note that I said they’re more economical at hovering. I wouldn’t be surprised if they didn’t score so well on a transatlantic flight. Which may be one reason why they don’t travel very far, unlike some birds.]

Offhand I can think of two sci-fi/fantasy creatures that run like a vampire bat: landstriders from The Dark Crystal and E.T. (in computer-generated footage from the 2002 version of the eponymous film).

Very cool vids!
That top one is a macrochiropteran or “flying fox”–it’s much larger body size (than the other microchiropterans depicted) probably explains most of the differences in flight mechanics (inc. leg use). Check out the tongue on the one that hovers at the plastic flower! And, the front-wheel-drive gait of the “running” vampire is technically, I think, a gallop (though it reminds me most of the human swimming stroke called butterfly).

Thanks a lot for posting this; fantastic footage! –however, I wouldn’t diss the long-distance flight capability of bats so quickly; the vast majority of (even very!) isolated tropical and subtropical oceanic islands have one or several species of large fruit bats, as well as (sometimes) several species of “microbats”. In fact, just with the last few years (in 2007) the island of Reunion has been re-colonised (they went extinct here) by Pteropus niger fruitbats crossing over from the neighbouring island (150 km away) of Mauritius! (where they are obviously still extant).

@Fred Magyar: Yes, that circular motion with the arms that the first bat makes, moving downwards and to the front, and then upwards and to the back, is exactly the one I used to make when treading water in the pool. The reason is (I think) it uses the trapezoids on the down-stroke, which maximises the lift, while also tending to spread/straighten the arms on the down-stroke and fold them slightly on the upstroke, which may also maximise lift and minimise drag.

Great post, Carl. Make.com has links to a plan for making a bat detector. It is basically the same electronics as the cereal box guitar amp. The bat detector picks up the bat’s chirp and turn it into something we can hear (through a small speaker) or see (flashing LED). I’m new to soldering and reading electronic circuit plans and tried to make the amp. I think I’ll get it this next time, then it’s on to the bat detector so my daughter and I can “see” bats in the dark.

This is fascinating. I’ve always been intrigued by bats. I have a question. I live in the Berkshires in Massachusetts. Last year they say it was the worst summer for mosquitos in over 20 years. Part of the reason was that we had a lot of rain, but many people said that the bat population was down considerably because of some sort of disease. Can you comment on this? They would attack anytime of day. Going outside for two minutes to pick some lettuce would get you 5 or 6 bites.
[Carl: There’s a terrifying fungus wiping out vast number of bats right now. Here’s a story from the Hartford Courant on the disaster here in Connecticut: 90% of the bats are dead. And since each bat eats 3,000 mosquitoes or moths a night, I am stocking up on DEET for the summer.]

The 5th video is a wonderful piece because it shows how a true expert flier will transform from using his/her wing top surface for lift to using the bottom surface for lift while upside down!!! The other video with the upside down landing looked like that bat used momentum to grab onto the grate. Thanks for posting this.

Let’s just make sure that everyone is real careful with that phrase, “squirrel-like”. While their putative ancestor may have been analogous to the modern squirrel in the niche they occupied, bats are NOT rodents. Among many other derived characteristics that they lack, the most obvious is the absence of Rodentia’s ever-growing incisors.

Of course, precisely where bats fall in the evolutionary tree is a little tricky and has been subject to a lot of recent revision. I think I remember hearing that recent molecular evidence now suggested their ancestors fall within the Laurasiatherian clade, nearer to the Insectivora and Carnivora. This is the sister group to the Euarchontoglires clade, which contains the Dermoptera, Rodentia, and Primates who were once suggested to be bats’ nearest relatives. I’d actually love to see an article on all of the recent revisions that have rumbled through the early-mammal community. There is a lot of great stuff showing work between molecular and fossil folks out there.

[Carl: How about squirrel-like-ish? You are absolutely right to point to the ongoing research to figure out just which living mammals are most closely related to bats. I merely meant that the non-flying ancestors of bats probably lived a lot like squirrels do today, a suggestion you can find in several recent papers on bat evolution.]

Thank you for this wonderful article, and for alerting people to WNS (White Nose Syndrome). There’s more information about this (and fabulous bats!) available at the Bat Conservation International page at: http://batcon.org/

I read an article a while back saying pterodacytl(or it could havebeen another ptero…) got up to take up speed by running on all fours using their strong front limbs for most of the power unlike birds which use their hind limbs only. This was supposed to explain how they could reach higher land speeds before taking off than previously thought. The vampire bat running video made it very clear how that could have worked.

I also enjoy watching bats and really enjoyed the videos and explainations. They really cleared up some things about bats. Particularly I didn’t know that they could hover more efficiently, I had always taken the description of bats “flying by brute force” to heart, but now I see that it isn’t an entirely fair description.

I love the way the Vampire bat uses it’s front legs/arms (his strongest legs). I love bats more than anything in the world! I even have two stuffed animals, but I like the first one. There was a big difference between how the bat ran and walked.

This post is really interesting to me, before I wasn’t too familar with bats and didn’t know what they did for us humans or the enviroment, but now I do since I have had to research and write articles on them for a project. I love to learn and read about them now..

Other people that have commented, as well as yourself, have touched on some interesting points. From my own observation and research I can see that bats fly all over the place because they have very poor vision, and obviously walking is no exception. Blind people use a stick or a golden retriever to assist them whereas bats are only able to really on themselves and the systems and lifestyles they have developed, and with ridiculously large wings for their body they are going to look even more like they are swimming rather than flying.

Different bat species develop in different ways, that’s the reason they walk differently and fly differently. I believe they are stable in their own right, however they may appear to be unstable when moving from place to place.

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Ed Yong is an award-winning British science writer. Not Exactly Rocket Science is his hub for talking about the awe-inspiring, beautiful and quirky world of science to as many people as possible.
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